This invention relates to gas turbine engines and particularly to means for controlling the clearance between the turbine outer air seal and the tip of the turbine rotor.
It is well known that the clearance between the tip of the turbine and the outer air seal is of great concern because any leakage of turbine air represents a loss of turbine efficiency and this loss can be directly assessed in loss of fuel consumption. Ideally, this clearance should be maintained at zero with no attendant turbine air leakage or loss of turbine efficiency. However, because of the hostile environment at this station of the gas turbine engine such a feat is practically impossible and the art has seen many attempts to optimize this clearance so as to keep the gap as close to zero as possible.
Although there has been external cooling of the engine case, such cooling heretofore has been by indiscriminately flowing air over the casing during the entire engine operation. To take advantage of this air cooling means, the engine case would typically be modified to include cooling fins to obtain sufficient heat transfer. This type of cooling presents no problem in certain fan jet engines where the fan air is discharged downstream of the turbine, since this is only a matter of proper routing of the fan discharge air. In other installations, the fan discharge air is remote from the turbine case and other means would be necessary to achieve gap control and this typically has been done by way of internal cooling.
Even more importantly, the heretofore system noted above that call for indiscriminate cooling do not maximize gap control because it fails to give a different clearance operating line at below the maximum power engine condition (Take-off). This can best be understood by realizing that minimum clearance occurs for maximum power since this is when the engine is running hottest and at maximum rotational speed. Because the casing is being cooled at this regime of operation the case is already in the shrunk or partially shrunk condition so that when the turbine is operating at a lower temperature and or lower speed the case and turbine will tend to contract back to their normal dimension. Looking at FIG. 2, this is demonstrated by the graph which is a plot of compressor speed and clearance.
It is apparent from viewing the graph that point A on line B is the minimum clearance and any point below will result in contact of the turbine and seal. Obviously, this is the point of greatest growth due to centrifugal and thermal forces, which is at the aircraft take-off condition at sea level. Hence, the engine is designed such that the minimum clearance will occur at take-off. Without implementing cooling, the parts will contract in a manner represented by line B such that the gap will increase as the engine's environment becomes less hostile. Curve C represents the gap when cooling is utilized.
It is apparent that since line C will result in a closure of the gap and rubbing of the turbine and seal as it approaches the sea level take-off operating regime, the engine must be designed so that this won't happen. Hence, with indiscriminate cooling, as described, line C would have to be moved upwardly so that it passes through point A at the most hostile operating condition. Obviously, when this is done operating of the engine will essentially provide a larger gap at the less hostile engine operating conditions.
We have found that we can obviate the problem noted above and minimize turbine air losses by optimizing the thermal control. This is accomplished by turning the flow of cool air on and off at a certain engine operating condition below the take-off regime. Preferably, maximum cruise would be the best point at which to turn on the cooling air. The results of this concept can be visualized by again referring to the graph of FIG. 2. As noted the minimum clearance is designed for take-off condition as represented by point A on curve B. The clearance will increase along curve B as the engine power is reduced. When at substantially maximum cruise, the cooling air will be turned to the on condition resulting in a shrinkage of the engine case represented by curve D. When full cooling is achieved, further reduction in engine power will result in additional contraction of the turbine (due to lower heat and centrifugal growth) increasing the gap demonstrated by curve C.
The on-off control is desirable from a standpoint of simplicity of hardware. In installations where more sophistication and complexity can be tolerated, the control can be a modulating type so that the flow of air can be modulated between full on and off to achieve a discreet thermal control resulting in a growth pattern that would give a substantially constant clearance as represented by the dash line E.
This invention contemplates a viable parameter that will effectuate the control of an on-off valve. We have found that a measurement of compressor speed is one such parameter and since this is typically measured by existing fuel controls, it is accessible with little, if any, modification thereto. As will be appreciated other parameters could serve a like purpose.